Frequency Control Software

For calculating VCO's frequency tuning range.
The resonant circuit is so much general, that it can be used for wide
range of VCOs.
The capacitance of the varactor diode is given at min. max. and center
tuning voltage.
For the inductive component a combination of series LC and a transmission line
can be given.
The transmission line can be specified by either electrical or mechanical (microstrip)
dimensions.

These are simple programs for 70 cm RX LO synthesizer. Frequency channels can be
changed by calculating new division ratios and editing the code.

These synthesizer ICs are common in older 450 MHz radiotelephones made by Nokia, which
are popular ham projects for 70 cm FM.

On connecting electronic circuits to parallel port

Some caution is in order when connecting external circuits to PC.
There is possibility, that voltage or current ratings are exceeded in some
circumstances, allthough breakdowns of devices are not very common.
In my opinion one should consider the following with every
connection.

If the grounds are not connected together, is there possibility, that high
voltages exist due to mains leak.

If the other side is off and other side in life, is there possibility
of too high current somewhere.

Generally it is recommended to keep your hands off the connector when power is on.
For overcurrent protection one frequently sees series resistors of 1k or more in lines,
when the parallel port is connected to CMOS logic. Another alternative is the interface
I used with AD9854.
These work if the data rate is not too high.

Old TTL was by nature quite robust, but nowadays there exists all kind of logic with
protection diodes, which may heavily load the driving stage.

Never seen a good treatise of this area by someone who knows, not just having read books.